Two controversial theses for the Langologists out there...

Lango-Django

It's possible to soften the timbre of rhythm comping without using the thumb--I do it all the time with a pick. Change the angle of attack, use the shoulder rather than point, play closer to the bottom of the fingerboard, don't play as hard, or any combination of these. And I wonder whether, given the conditions of early electrical recording (probably a single mike) and Lang's guitar setup, whether thumbing would have cut it.

OK, excellent point, Russell.

Jazzaferri

On mY Gretsch, I can hear a difference in attack between pick and thumb even when I soften it a bit more than I normally play electric, which is still pretty soft attack. It's evident on a solo'd track or if I am soloing but buried in the mix can't really tell the diff. FWIW